Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system

The present study was carried out to investigate whether the P concentration in the roots or the shoots controls the growth and citrate exudation of cluster roots in white lupin (Lupinus albus L). Foliar P application indicated that low P concentration in the shoots enhanced cluster-root growth and citrate-exudation rate more so than low P concentration in the roots. In the split-root study, the P concentration in the shoots increased with increased P supply (1, 25 or 75 mmol m(-3) P), to the 'privileged' root halves. Roots 'deprived' of P invariably had the same low P concentrations, whereas those in the 'privileged' roots increased with increasing P supply (1, 25 or 75 mmol m(-3) P). Nevertheless, the proportion of the total root mass allocated to cluster roots, and the citrate-exudation rates from the root halves were always similar on both root halves, irrespective of P supply, and decreased with increasing shoot P concentrations. Peak citrate exudation rates from developing cluster roots were significantly faster from cluster roots on the 'deprived' root halves when the 'privileged' half was exposed to 1 mmol m(-3) P as compared with 25 or 75 mmol m(-3) P. The possibility that changes in the concentrations of P fractions in the root halves influenced cluster-root growth and citrate exudation was discounted, because there were no significant differences in insoluble organic P, ester-P and inorganic P among all 'deprived' root halves. The results indicate that cluster-root proportions and citrate exudation rates were regulated systemically by the P status of the shoot, and that P concentrations in the roots had little influence on growth and citrate exudation of cluster roots in L. albus .